Current Inability to Expand and Distract Endplates
A conventional spine cage or implant is characterized by a kidney bean shaped body comprising a hydroxyapetite coated surface provided on the exterior surface for contact with adjacent vertebral segments or endplates which are shown in FIG. 1. A conventional spine cage is typically inserted in tandem posteriorly through the neuroforamen of the distracted spine after a trial implant creates a pathway.
Such existing devices for interbody stabilization have important and significant limitations. These limitations include an inability to expand and distract the endplates. Current devices for interbody stabilization include static spacers composed of titanium, PEEK, and high performance thermoplastic polymer produced by VICTREX, (Victrex USA Inc, 3A Caledon Court; Greenville, S.C. 29615), carbon fiber, or resorbable polymers. Current interbody spacers do not maintain interbody lordosis and can contribute to the formation of a straight or even kyphotic segment and the clinical problem of “flatback syndrome.” Separation of the endplates increases space available for the neural elements, specifically the neural foramen. Existing static cages do not reliably improve space for the neural elements. Therefore, what is needed is an expanding cage that will increase space for the neural elements posteriorly between the vertebral bodies, or at least maintain the natural bone contours to avoid neuropraxia (nerve stretch) or encroachment.
Poor Interface between Bone and Biomaterial
Another problem with conventional devices of interbody stabilization includes poor interface between bone and biomaterial. Conventional static interbody spacers form a weak interface between bone and biomaterial. Although the surface of such implants is typically provided with a series of ridges or coated with hydroxyapetite, the ridges may be in parallel with applied horizontal vectors or side-to-side motion. That is, the ridges or coatings offer little resistance to movement applied to either side of the endplates. Thus, nonunion is common in allograft, titanium and polymer spacers, due to motion between the implant and host bone. Conventional devices typically do not expand between adjacent vertebrae.
Therefore, what is needed is a way to expand an implant to develop immediate fixation forces that can exceed the ultimate strength at healing. Such an expandable implant ideally will maximize stability of the interface and enhance stable fixation. The immediate fixation of such an expandable interbody implant advantageously will provide stability that is similar to that achieved at the time of healing. Such an implant would have valuable implications in enhancing early post-operative rehabilitation for the patient.
Conventional Large Size Static Devices Require Wide Exposure of Neural Structures
Another problem of conventional interbody spacers is their large diameter requiring wide exposure. Existing devices used for interbody spacers include structural allograft, threaded cages, cylindrical cages, and boomerang-shaped cages. Conventional devices have significant limitation with regard to safety and efficacy. Regarding safety of the interbody spacers, injury to neural elements may occur with placement from an anterior or posterior approach. A conventional spine cage lacks the ability to expand, diminishing its fixation capabilities.
The risks to neural elements are primarily due to the disparity between the large size of the cage required to adequately support the interbody space, and the small space available for insertion of the device, especially when placed from a posterior or transforaminal approach. Existing boomerang cages are shaped like a partially flattened kidney bean. Their implantation requires a wide exposure and potential compromise of vascular and neural structures, both because of their inability to enter small and become larger, and due to the fact that their insertion requires mechanical manipulation during insertion and expanding of the implant. Once current boomerang implants are prepared for insertion via a trial spacer to make a pathway toward the anterior spinal column, the existing static cage is shoved toward the end point with the hope that it will reach a desired anatomic destination. Given the proximity of nerve roots and vascular structures to the insertion site, and the solid, relatively large size of conventional devices, such constraints predispose a patient to foraminal (nerve passage site) encroachment, and possible neural and vascular injury.
Therefore, what is needed is a minimally invasive expanding spine cage that is capable of insertion with minimal invasion into a smaller aperture. Such a minimally invasive spine cage advantageously could be expanded with completely positional control or adjustment in three dimensions by hydraulic force application through a connected thin, pliable hydraulic line. The thin hydraulic line would take the place of rigid insertional tools, thereby completely preventing trauma to delicate nerve endings and nerve roots about the spinal column. Due to the significant mechanical leverage developed by a hydraulic control system, the same expanding cage could advantageously be inserted by a minimally sized insertion guiding rod tool capable of directing the cage through the transforaminal approach to a predetermined destination, also with reduced risk of trauma to nerve roots. That is, the mechanical advantage is provided by a hydraulic control system controlled by the physician external to the patient.
The minimally sized insertion tool could house multiple hydraulic lines for precise insertion and expansion of the cage, and simply detached from the expanded cage after insertion. It is noted that in such a hydraulic system, a smaller, thinner line advantageously also increases the pounds per inch of adjusting force necessary to achieve proper expansion of the implant (as opposed to a manually powered or manipulated surgical tool) that must apply force directly at the intervention site. That is, for a true minimally invasive approach to spinal implant surgery what is needed is an apparatus and method for providing the significant amount of force necessary to properly expand and adjust the cage against the vertebral endplates, safely away from the intervention site.
What is also needed is a smaller expanding spine cage that is easier to operatively insert into a patient with minimal surgical trauma in contrast to conventional, relatively large devices that create the needless trauma to nerve roots in the confined space of the vertebral region.
Limited Capacity for Interbody Bone Formation
Existing interbody implants have limited space available for bone graft. Adequate bone graft or bone graft substitute is critical for a solid interbody arthrodesis. It would be desirable to provide an expandable interbody cage that will permit a large volume of bone graft material to be placed within the cage and around it, to fill the intervertebral space. Additionally, conventional interbody implants lack the ability to stabilize endplates completely and prevent them from moving. Therefore, what is also needed is an expanding spine cage wherein the vertebral end plates are subject to forces that both distract them apart, and hold them from moving. Such an interbody cage would be capable of stabilization of the motion segment, thereby reducing micromotion, and discouraging the pseudoarthrosis (incomplete fusion) and pain.
Ideally, what is needed is a spine cage or implant that is capable of increasing its expansion in width anteriorly to open like a clam, spreading to a calculated degree. Furthermore, what is needed is a spine cage that can adjust the amount of not only overall anterior expansion, but also medial and lateral variable expansion so that both the normal lordotic curve is maintained, and adjustments can be made for scoliosis or bone defects. Such a spine cage or implant would permit restoration of normal spinal alignment after surgery and hold the spine segments together rigidly, mechanically, until healing occurs.
What is also needed is an expanding cage or implant that is capable of holding the vertebral or joint sections with increased pullout strength to minimize the chance of implant fixation loss during the period when the implant is becoming incorporated into the arthrodesis bone block.
It would also be desirable if such a cage could expand anteriorly away from the neural structures and along the axis of the anterior spinal column, rather than uniformly which would take up more space inside the vertebral body surfaces.